This invention relates to alkali metal and ammonium salts of crosslinked phosphonoalkyl cellulose and to absorbent devices incorporating the same.
Cellulose is often used as an absorbent material in a variety of absorbent devices such as catamenial tampons, sanitary napkins, diapers, and the like. Although cellulose is an adequate absorbent material for many purposes, increased use of disposable absorbent devices has increased the demand for materials which are more absorbent than cellulose. Attention has therefore been directed to techniques for chemically or physically modifying cellulose to increase its absorbency.
One technique which has been used in an attempt to modify cellulose to render it more absorbent is that of substituting ionizable groups for the hydroxyl groups on the anhydroglucose units of cellulose. Such groups render the modified cellulose more absorbent than unmodified cellulose because of their affinity for water, but this affinity also tends to make the modified cellulose more water-soluble than unmodified cellulose. The greater the degree of substitution of the ionizable groups, the greater the degree of water-solubility of the resulting modified cellulose. Finally, the point is reached at which the material is completely soluble in aqueous fluids at ambient temperature and thus useless as an absorbent material. The utility as an absorbent material of substituted cellulose of this type which is otherwise unmodified is severely limited by this increase in solubility.
Thus, while the preparation of phosphonomethyl cellulose was disclosed in Drake, et al., U.S. Pat. No. 2,979,374, the inventors there teach its main utility to be in reducing the combustability of fibers. They disclose that the material becomes readily water-soluble if it contains at least about 2% by weight phosphorous but may also be soluble at lesser concentrations, depending on the form of cellulose used. Because of this solubility of phosphonomethyl cellulose, Drake, et al., do not teach any use thereof as an absorbent material.
It has now been discovered that the undesirable solubility of phosphonoalkyl cellulose (including phosphonomethyl cellulose) in aqueous fluids may be reduced essentially to zero while retaining the desirable absorptivity by crosslinking the phosphonoalkyl cellulose chains. This crosslinked phosphonoalkyl cellulose is essentially insoluble in aqueous fluids and yet has much greater absorbency than unmodified cellulose.